In hot metal dip coating processes for metals such as zinc and aluminum and in the coated paper and film industries, the thickness of the liquid coating remaining on the metal strip, film or paper web, all three being herein sometimes generically referred to as webs, must be controlled to obtain a satisfactory product. One of the difficulties encountered in obtaining a web with uniform liquid coating thickness is the occurrence of heavy edge coating, a phenomenon that has become known as edge effect. In this phenomenon, due to the complexities of fluid flow from the opposing nozzles at the edges of the web, the liquid coating remaining on the web is heavier close to and along the edges or marginal portions of the web. Much effort has been expended in attempting to eliminate this difficulty.
U.S. Pat. Nos. proposing solutions for eliminating edge effect are 3,406,656, 3,480,469, 3,526,203, 3,670,695, 3,672,324, 3,687,103, 3,742,905, 3,773,013.
In addition to the waste of coating material and other problems which result from edge effect in the coated metal paper and film environments, in the metal coating industry where the strip metal such as steel is coated with coating metal such as zinc and aluminum a further very serious difficulty arises from edge effect. The coated metal strip is coiled as it is produced for convenience in handling and shipping. The slightly thicker coating metal at the edges of the strip results in "spooling" in the usual large coil. In spooling the strip assumes a concave configuration as the coil builds up. Spooling is a very serious defect because it can cause the strip edges to be stretched plastically resulting in a wavy edge when the strip is uncoiled. Such strip is commercially unacceptable.
Several of the above cited U.S. patents attempt to explain the phenomena which result in edge effect in strip or web coating. Regardless as to what explanations are correct, applicant has discovered that by impinging against a moving web coated with liquid a stream of fluid which has a component of motion toward each edge of the strip or web, edge effect in the finally coated product can be greatly reduced or even entirely eliminated and for practical purposes substantially uniform coating thickness obtained across the width of the web.